Field of the Invention
The present invention relates to a deformable mirror unit capable of correcting the wavefront error and distortion of an optical system in an exposure apparatus, astronomical telescope, or the like, and an exposure apparatus.
Description of the Related Art
Recently, as a demand on the resolution of an exposure apparatus is becoming stricter, a demand for correcting aberration induced by exposure is also becoming stricter. To correct aberration induced by exposure, there has been proposed an arrangement using a deformable mirror.
Japanese Patent No. 4361269 discloses a mirror support structure in which an outer ring supports a reaction assembly to which a pneumatic actuator for deforming a mirror is attached, and an inner ring supporting the mirror. Japanese Patent Laid-Open No. 2004-64076 discloses a servo control mechanism including many actuators which support a deformable mirror at least three rigid positions, and displacement sensors located near them.
In Japanese Patent No. 4361269, the pneumatic actuator is mechanically coupled to the rear surface of the mirror via a rod, and thus excessively constrained with respect to the mirror. The mirror shape is readily affected by an assembly error. In the mirror support structure disclosed in Japanese Patent No. 4361269, the reaction force of the pneumatic actuator is transferred to the mirror via the reaction assembly, inner ring, and outer ring, and may affect the mirror shape.
Japanese Patent Laid-Open No. 2004-64076 has proposed a mirror unit with a displacement feedback driving control system including many actuators and displacement sensors located near them. The mirror unit with the displacement feedback driving control system measures the displacement of the mirror at or near each driving point, and controls the actuators so that the displacement coincides with a target displacement value in a given accuracy range. Hence, the mirror unit is free from the influence of a driving reaction force. However, this mirror unit has a complicated arrangement, requires many displacement sensors, and is expensive.